Tube diameter of oriented and stretched polymer melts

The Lin-Noolandi entanglement ansatz correlates the tube diameter with polymer density and chain stiffness, and has been successfully extended to concentrated solutions as well. We develop a new version of the entanglement ansatz, applicable to polymer melts under uniform tension, and find that the tube diameter equals its unperturbed value a when the pulling force F is less than the thermal tension kT/a, and scales as F^-1/3 when F exceeds kT/a. We verify this behavior by using isoconfigurational ensemble averaging to estimate the tube diameter in simulated polymer melts that are oriented by pulling on the ends of topologically equilibrated chains. When the tension is large enough to nearly fully extend the chain, our analysis predicts that the tube diameter scales as F^-2/3, but our simulation results do not reach this regime.